There is a continuous development of new generations of mobile communications technologies to cope with increasing requirements of higher data rates, improved efficiency and lower costs. High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are mobile communication protocols that were developed to cope with higher data rates than original Wideband Code Division Multiple Access (WCDMA) protocols were capable of. The 3rd Generation Partnership Project (3GPP) is a standards-developing organization that is continuing its work of evolving HSPA and creating new standards that allow for even higher data rates and improved functionality.
In a radio access network implementing HSPA, a user equipment (UE) is wirelessly connected to a base station commonly referred to as a NodeB (NB). A base station is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
Recently 3GPP has started a number of work items targeting uplink multi-antenna solutions for a standard release 11. In particular, there are work items targeted for open and closed loop uplink transmit diversity as well as a study item on uplink multiple-input-multiple-output (MMO) transmission.
With uplink transmit diversity, UEs that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. This is achieved by multiplying a UE output signal with a set of complex pre-coding weights, a so-called pre-coding vector with one pre-coding weight for each physical transmit antenna. The rationale behind uplink transmit diversity is to adapt the pre-coding weights so that user equipment and network performance is maximized. Depending on UE implementation the antenna pre-coding weights may be associated with different constraints. Within 3GPP two classes of transmit diversity are considered:                Switched antenna transmit diversity, where the UE at any given time-instance transmits from one of its antennas only.        Beamforming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beamforming it is possible to shape an overall antenna beam in the direction of a target receiver. It can be noted that switched antenna transmit diversity can be seen as a special case of beamforming where one of the pre-coding weights is 1 (i.e. switched on) and the pre-coding weight of any other antenna of the UE is 0 (i.e. switched off).        
Transmit diversity schemes can be seen as a generic framework for mapping symbols to antenna ports. With respect to beamforming techniques, rank-1 transmissions, the same symbols are mapped to the several physical antennas and by adapting the pre-coding vector so that it matches the “radio” channel the UE and network performance, e.g., coverage, throughput, transmit power, etc., can be improved. More specifically, this is achieved by multiplying the signal with a set of complex weights wi, one for each physical antenna. Mathematically, this can be written as follows
      [                                        y            1                                                ⋮                                                  y            N                                ]    =                    [                                                            w                1                                                                        ⋮                                                                          w                N                                                    ]            ⁢                          ⁢      s        =    ws  where y1 is the signal at the /th antenna port and where w is typically referred to as the pre-coding or beamforming vector. As noted above the fundamental idea with uplink transmit diversity is to exploit the properties in the effective channel and ensure that coherent combining is achieved at the receiver. The term effective channel here incorporates effects of transmit and receive antennas as well as the radio channel between the transmitting and receiving antennas.
For uplink MIMO, different data is transmitted from different virtual antennas in so-called streams. Each virtual antenna corresponds to a different pre-coding vector. Note that closed loop beamforming can be viewed as a special case of uplink MIMO where no data is scheduled on all but one of the possible virtual antennas.
MIMO technology is mainly beneficial in situations where the “composite channel” is strong and has high rank. The term composite channel includes the potential effects of transmit antenna(s), power amplifiers (PAs), as well as the radio channel between the transmitting and receiving antennas. The rank of the composite channel depends on the number of uncorrelated paths between the transmitter and the receiver. Single-stream transmissions, i.e. beamforming techniques, are generally preferred over MIMO transmissions in situations where the rank of the composite channel is low e.g. where there is a limited amount of multi-path propagation and cross polarized antennas are not used, and/or the path gain between the UE and the NodeB is weak.
In closed-loop techniques, such as closed loop transmit diversity or uplink MIMO, the network decides or recommend the pre-coding vector(s) that the UE should apply by means of a physical channel. One example of such a physical channel would be to rely on a Fractional Dedicated Physical Channel (F-DPCH)-like channel or a grant channel, such as an Enhanced Dedicated Channel Absolute Grant Channel (E-AGCH)-like or an Enhanced Dedicated Channel Relative Grant Channel (E-RGCH)-like channel. A serving NodeB could e.g. signal a recommended pre-coding vector to a UE on the physical channel by means of an explicit or implicit indication of the recommended pre-coding vector(s).
The terms “pre-coding vector information”, “pre-coding information” and “feedback information” are used synonymously herein to refer to information transmitted on the above mentioned physical channel in order for the network to indicate pre-coding weights/one or several pre-coding vectors that a UE is recommended to use for uplink transmission. Pre-coding vector information that recommends pre-coding vector(s) to a UE may also be referred to as a “pre-coding command”. The term “pre-coder” is used herein to refer to a set of pre-coding weights which can comprise one or several pre-coding vectors.
Regardless of the physical channel that is used to signal feedback from the network to the UE one key aspect of closed-loop schemes is that a sufficient reception quality of the feedback quality can be ensured. In fact as the reception quality of the physical channel carrying the feedback information deteriorates the reliability of the signaled pre-coding vector(s) becomes increasingly unreliable and at some point the usage of closed loop beamforming or uplink MIMO may become harmful. This is because the UE will, in case of unreliable pre-coding commands, start to direct its beam in a random direction which may cause excessive amounts of interference in neighboring cells. Moreover, the variations in interference will become more rapid. Also, the performance and Rise-over-Thermal (RoT) utilization may be reduced because the received power from a given UE will experience faster variations due to the rapid changes in the pre-coding weights. Consequently, inner loop power control may be unable to track the channel at low Doppler spreads.
Another design choice related to closed loop transmit diversity schemes and MIMO schemes is the physical channel structure and more specifically which physical channels that should be multiplied with the pre-coding vector. According to an example architecture one of the pilots transmitted on a Dedicated Physical Control Channel (DPCCH) and all of the other physical channels are transmitted using a certain pre-coding vector, often referred to as the primary pre-coding vector. The other pilot transmitted on the other DPCCH is transmitted with another pre-coding vector, which e.g. is orthogonal to the primary pre-coding vector. According to an alternative example architecture the pilots are non-precoded and the pre-coding is only applied to the data related channels. Note that 3GPP is considering an architecture based on pre-coded pilots for closed loop transmit diversity. One of the main benefits with an architecture based on pre-coded DPCCH pilots is that the pre-coding vector that the UE has applied does not need be known by the network. This is because channel estimates used for demodulation can be on the pre-coded DPCCH pilot that is pre-coded in the same way as the data channels.
For closed loop transmit diversity and UL MIMO, it is important that the reception quality of the physical channel carrying the pre-coding information can be maintained at a reasonable level. It is also desirable that the UE does not update its pre-coding vector too frequently, if this is not necessary, because frequent updates may cause excessive interference levels in surrounding cells as well as harm the user performance and RoT utilization efficiency at a NodeB controlling the pre-coding vector generation. Furthermore, too frequent updates of the applied pre-coding vectors will make it harder for the inner loop power control (ILPC) loop to adapt to the effective pre-coded channel, which in general will exhibit larger variations than a raw radio channel if a “random” pre-coder is applied.
Thus there is a desire for schemes that help to avoid harmful situations relating to pre-coding vector updates in communication systems applying closed loop multi-antenna transmission techniques such as closed loop uplink transmit diversity and uplink MIMO.